The famous narration of the Prophet, the Hadith of Thaqalayn, is narrated in different versions by all Islamic schools of thought with regards to the Ahl al-Bayt. The Prophet may have mentioned it on different occasions due to its importance and to ensure that many can hear it:

O People, I leave among you two precious things: the book of God and my household. As long as you hold on to them, you will not go astray.

In another version, the Prophet ordered a similar command:

I leave among you two precious things. If you hold on to them, you will not go astray after me: the book of God, which is like a rope extended between the heaven and the earth, and my household. These two will not separate from each other until they reach me near the fountain of Kawthar on the Day of Judgment. Be alert with how you treat them after me.1

I leave among you two precious things. If you hold on to them, you will not go astray after me: the book of God, which is like a rope extended between the heaven and the earth, and my household. These two will not separate from each other until they reach me near the fountain of Kawthar on the Day of Judgment.

In the mid-1900s, Grand Ayatollah Borujerdi and the Shaykh of al- Azhar University, Mahmud Shaltut, established a Shi‘a-Sunni journal for Dar al-Taghrib Bayn al-Madhahib al-Islamiyyh called Risalat al-Islam published in Cairo.
In the first issue of this journal, an article by a Sunni scholar was published in which the author had used the hadith “I left you the Thaqalayn: the book of God and my sunnah” instead of ‘Itrati, my progeny, which is narrated by all Muslims. Upon reading the article, Ayatollah Borujerdi asked a scholar in Qum to collect all the Sunni references for “the Book of God and my progeny.”
As a result, a booklet containing 200 Sunni sources was produced. Ayatollah Borujerdi then sent this to the scholars at the al-Azhar in Cairo who published it. Thus, the hadith of Thaqalayn is an indisputable and well-established hadith.
Of course, even if we accept the version “the book of God and my Sunnah” which is narrated by only a few Sunni scholars, it will not cause any problem, because at best it can be taken equal to the version “the book of God and my household” and once the Prophet (s) never speaks in vain or in a contradictory way, the overall result would be that we need to refer to the book of God and the Sunnah of the Prophet in the way the household of the Prophet (s) have reported us.
If the Prophet in many cases said “Kitab Allah wa ‘Iitrati” and in some cases “Kitab Allah wa Sunnati” kitab Allah is the same in both hadiths, so sunnah and ‘Itra of the Prophet must be harmonious too. Thus, to be able to understand the Sunnah you must refer to the Ahl al-Bayt.

If the Prophet in many cases said “Kitab Allah wa ‘Iitrati” and in some cases “Kitab Allah wa Sunnati” kitab Allah is the same in both hadiths, so sunnah and ‘Itra of the Prophet must be harmonious too. Thus, to be able to understand the Sunnah you must refer to the Ahl al-Bayt. One cannot believe in sunnah alone.

You need to refer to the Ahl al-Bayt to find the true sunnah. In any case, the following points can be inferred from the hadith of thaqalayn:
1) The book of Allah and the Ahl al-Bayt must be available until the end of this world. The Qur’an will always be available; thus, the Ahl al-Bayt must also continue. This is so because the Prophet said that if people appeal to both, they will never go astray until they meet him near the fountain of Kawthar. Thus, if the book of Allah or the Ahl al-Bayt (God forbid) is missing, how can we refer to them? Both must be available, not just one.

The Ahl al-Bayt must be there to interpret the Qur’an because among the seventy-three sects (which emerged after the demise of the Prophet) exist various debates on the interpretation of the Qur’an. For example, in a dispute about the visibility of God, the Ash‘arites believed it is possible to see God with the physical eye.

2) No one can say the book of God is enough by itself. When the Prophet wanted to dictate something while in his deathbed, he asked for something to write with and gave his reason (“So that you will not go astray”). The second caliph, Umar, was present, and according to Sunni sources, he said that the Prophet is not feeling well and does not have full control of what he is saying.
He then said, “The book of Allah is sufficient,” meaning that we do not need the instruction of the Prophet (s). However, according to the Qur’an itself, there must be someone as a teacher of the Qur’an. The Prophet (s) was the first one to teach the Qur’an to people and then this task was handed over to the Ahl al-Bayt and therefore the Prophet asked everyone to hold onto both the Qur’an and the Ahl al-Bayt.
The Ahl al-Bayt must be there to interpret the Qur’an because among the seventy-three sects (which emerged after the demise of the Prophet) exist various debates on the interpretation of the Qur’an. For example, in a dispute about the visibility of God, the Ash‘arites believed it is possible to see God with the physical eye.2
And they argued from the verse of the Qur’an which states,

“Some faces will be fresh on that day, looking at their Lord” (75:22-23).

Muslims such as the Shi‘a and Mu‘tazilites who disagree with the Ash‘arites argued from the Qur’an as well. Thus, there must be an interpreter of the Qur’an along with the Qur’an that can speak on behalf of the Qur’an to people.

3) The Ahl al-Bayt must be infallible. The Prophet’s mention of the Ahl al-Bayt and the Qur’an as two things that will never separate from each other proves their infallibility. If the Ahl al- Bayt made mistakes, that would mean they are separate from the Qur’an. People cannot simultaneously appeal to a perfect Qur’an and imperfect leaders.

The Ahl al-Bayt must be infallible. The Prophet’s mention of the Ahl al-Bayt and the Qur’an as two things that will never separate from each other proves their infallibility. If the Ahl al- Bayt made mistakes, that would mean they are separate from the Qur’an. People cannot simultaneously appeal to a perfect Qur’an and imperfect leaders.

Is it wise of the Prophet to ask us to unconditionally follow those who may make mistakes and sin? The Ahl al-Bayt will never go astray; this is why we can appeal to them. Moreover, we can be saved by following them. They must always be in line with the Qur’an, and since the Qur’an is the constant truth, the teachings of the Ahl al-Bayt must also be true.
Also, since the Qur’an is preserved by Allah until the end of the world, Ahl al-Bayt must have also been given a sort of continuity by Allah. This is why we believe that there must be a living member of the Ahl al-Bayt on this earth.

• 1. These hadiths are mentioned in many Sunni sources. For example, see Sahih of Muslim, vol. 8, p. 25, number 2408, Musnad, No. 10720, Sunan of Darimi, vol. 2, p. 432, Sahih of Tirmidhi, vol. 5, p. 6432, No. 3788. It is also found in other books, such as Usd al-Ghabah (the Lions of the Forest) on the biographies of the Companions of the Prophet, Al-Sunan al-Kubra and Kanz al-‘Ummal.
• 2. Ash’arites: followers of a theology founded by Abu al-Hasan al-Ash’ari in the mid-900s